This invention relates to a method and apparatus for transmitting signals for an Extended Definition Television System (EDTV) for reception on standard NTSC television receivers with minimal interference from the enhancement portion of the EDTV signal. The described EDTV video transmissions are imaged in letter box format when received by a typical NTSC television receiver.
The current operating standard for television transmissions in the United States is defined by the National Television Systems Committee (NTSC). This standard was approved for monochrome transmission by the Federal Communications Commission in 1941 and further expanded for NTSC color television systems in 1954.
Newer systems associated with High Definition Television (HDTV) set higher standards of resolution for the transmission of television images as compared to the NTSC standard. While the HDTV standard will deliver a better TV image, the transmission of this HDTV image is generally incompatible with existing NTSC receivers. Therefore, a migration path to transition to this better quality HDTV image in the face of the large existing installed base of incompatible NTSC receivers has to be found. It is clearly desirable to transition from the NTSC system to the HDTV system in a smooth, gradual way where an intermediate standard compatible with both the NTSC and the HDTV systems allows use of the existing NTSC receivers while concurrently providing some the benefits of the higher image quality of the HDTV transmissions.
Such a system, designed as an intermediate step to minimize the impact of the transition to HDTV, is generally identified as a "letter box" Extended Definition Television (EDTV). A letter box type EDTV transmission presents an image on a standard NTSC receiver that contains the information related to the full width of the originally transmitted EDTV image and maintains the original 9 by 16 aspect ratio of the EDTV image. To fit the 9 by 16 aspect ratio EDTV image into the 3 by 4 aspect ratio of the NTSC display requires the introduction of top and bottom bands in the display. The EDTV transmission reproduced by the NTSC receiver must therefore accommodate these bands.
The bands are ideally black and uniform, and appear above and below the active image area displayed by the NTSC-only compatible receiver. Geometric considerations of displaying a 9 by 16 ratio image into a 3 by 4 aspect ratio display dictate that 180 lines per field be used by the NTSC receiver to display the EDTV image. Since the NTSC has a total of 240 display lines, the 60 remaining lines are divided into two groups, with 30 lines forming the upper band and 30 lines forming the lower band. It is important that these bands be uniform and non-changing so as not to interfere with viewing the 180 line EDTV image on the NTSC display.
The letterbox EDTV system generally transmits the added, or enhancement information required by the full EDTV image within the time associated with the top and bottom band areas around the NTSC image just before and after the transmission of the NTSC compatible portion of the image. Such letter box type system have been described for example by Norihiro Suzuki et al. in "Experimental Hardware for Proposed Letter Box Wide Aspect EDTV", SMPTE Technical Conference, October 1990, New York City, N.Y. and W. F. Schreiber et al., "Single Channel HDTV System, Compatible and Non-compatible", presented at the 2nd HDTV workshop, Italy, March 1988, as well as Y. Kanatsugu et al., "Development of MUSE Family Systems", IEEE Transactions of Consumer Electronics, Vol. 35, No. 3, August 1989, pp. 153-158.
In a typical letterbox EDTV system, the EDTV video source signal originally contains 480 lines of video information per frame, split into two components. The first, or low pass (NTSC component) signal contains 360 video lines per frame (180 lines per field). The second, or high pass (EDTV enhancement) signal portion contains 120 video lines per frame (60 lines per field) plus blanking (22.5 lines per field).
A problem exists when the EDTV signal is viewed on an NTSC only receiver. The EDTV related information present in high pass components (i.e., the top and bottom bands containing the information necessary to enhance the quality of the EDTV image) can occasionally modulate the display lines of the bands in the NTSC receiver so that the bands are non-uniform in appearance. This modulation can produce, in the band areas of the NTSC receiver, speckles or wavy lines that detract from the enjoyment of the NTSC image and interfere with the quality of the NTSC image displayed on the NTSC receiver.
The undesirability of having the NTSC receiver display even faintly visible marks in the top and bottom band areas has led to proposals for systems to "blank" those areas. For example, in the system described in "Matrix Conversion for Improvement of Vertical-Temporal Resolution in Letter Box Wide-Aspect TV" by N. Suzuki at SMPTE Journal, February 1991, the amplitude of the output of the high pass filter from the video transmitter is compressed to limit the dynamic range of the high frequency signals and therefore reduce the visibility of the modulation of the bars.
Another system for reducing the interference associated with the top and bottom bands is described in "Transmission Method of Side-Panel Signals--A Technique to Hide Side-Panel Signals" by Ashibe et al. of the NEC Corporation, Japan, which proposes to split the high pass information normally associated with the bands into a high and a low energy group. The low energy group is transmitted during the duration of the bands while the high energy group is transmitted during the normal overscan period of the NTSC receiver when the beam forming the image is not visible on the display of the receiver. This allocation of the high energy group signals at a time when the display is not produced reduces the probability that some of the EDTV enhancement signal may inadvertently modulate the NTSC receiver during the top or bottom band forming period.
In view of the present limitations associated with EDTV television systems, it is an objective of the present invention to provide an EDTV letter box system whose transmission of EDTV signals is such that when viewed on an NTSC receiver, the upper and lower bands have a uniform appearance unmarred by the unwanted display of enhancements related to the EDTV image.